1. Field of the Invention
The present invention relates to a signal processing circuit, and more particularly, to a signal processing circuit having a plurality of frequency synthesizers.
2. Description of the Related Art
In wireless local-area network (WLAN) applications, in order to achieve higher transmission speed and better transmission quality, the signal processing circuit often supports a multiple-input multiple-output (MIMO) technique. The MIMO technique uses multiple antennas to receive (and transfer multiple signal streams.
In addition, in order to increase the transmission speed, a common solution implemented is to increase the transmission bandwidths of the signals. For example, in a traditional WLAN system, the signal bandwidth is 20 MHz. However, WLAM systems can often support 40 MHz-bandwidth data transmission.
Please refer to FIG. 1. FIG. 1 is a diagram of a conventional signal processing circuit 100. As shown in FIG. 1, the signal processing circuit 100 comprises four antennas 111 through 114, an RF circuit 121, four analog front ends (AFE) circuits 131 through 134, a baseband circuit 141, and a synthesizer 151. In this case, each of the antennas 111 through 114 can receive and transfer an independent signal stream so as to support the above-mentioned MIMO technique. Please note, the functions and operations of the antennas 111 through 114, the RF circuit 121, the AFE circuits 131 through 134, and the baseband circuit 141 are well-known by those having average skill in the art, and therefore further illustrations are omitted herein.
Moreover, the RF circuit 121 can only use a single central frequency f (i.e., carrier frequency) to perform the data transmission regardless of the selected signal bandwidth (e.g., the aforementioned 20 MHz or 40 MHz) because the signal processing circuit 100 comprises only one synthesizer 151.
Please refer to FIG. 2. FIG. 2 is a simplified diagram showing output signals of the signal processing circuit 100 as shown in FIG. 1. When the signal processing circuit 100 transfers signals in a 40 MHz bandwidth, the baseband circuit 141 can only shift the frequency of the signal to be transferred 10 MHz right or left because of the limitation of having only the single synthesizer 151. In this way, the signals to be transferred change to correspond to two adjacent 20 MHz bandwidths. Next, the RF circuit 121 utilizes the central frequency f provided by the synthesizer 151 such that the signals can be carried in the 40 MHz bandwidth having the central frequency f. In this way, the signal processing circuit 100 can use the 40 MHz bandwidth instead of the original 20 MHz bandwidth to transfer data.
However, the above-mentioned structure, the single synthesizer structure, has several disadvantages. First, when the RF circuit 121 receives signals, a filtering operation is performed to derive desired signals. Please refer to FIG. 3. FIG. 3 is a diagram showing a frequency spectrum of the signal processing circuit 100 when the signal processing circuit 100 is receiving signals. As shown in FIG. 3, when the RF circuit 121 receives signals carried by the 40 MHz bandwidth, the RF circuit 121 can only set the central frequency of the filtering bandwidth as the central frequency f provided by the synthesizer 151 and the filtering bandwidth can only be 40 MHz because of the limitation of having just the single synthesizer structure. In this way, signals transferred in the two 20 MHz bandwidths interfere each other by their images (i.e., in the signal processing procedure, one signal becomes a noise for the other signal). Therefore, in the following signal analysis (e.g., in the following down-sampling operation performed by the AFE circuits 131 through 134), the processed signals may include distortions.
Furthermore, because of the limitation of having the single synthesizer structure, the signal processing circuit 100 cannot detect whether the adjacent channel is clean before switching the signal bandwidth. Therefore, when the signal processing circuit 100 changes to use the 40 MHz bandwidth to transfer data, packets to be transferred and other currently-transferring packets may encounter collisions. If the collision phenomenon is serious, the data throughput may be reduced. Therefore, it is apparent that new and improved methods and devices are needed.